| The first total hip replacement using a joint socket lined with a novel material invention has been performed by surgeons at Massachusettes General Hospital (MGH). The material is an advanced first-generation highly cross-linked polyethylene and significantly reduces a serious complication of early hip implants. The new material may be applied in replacements for a wider variety of joints in a more diverse group of patients, even those demanding higher flexion and more mobility.
Total replacements for hips and other joints were developed in the late 1960s, but it soon became apparent that hip implants could start loosening about 5 years after surgery and would eventually fail completely, mostly due to long-term friction of the implant's head against the polyethylene-lined joint socket that would break off small particles of polyethylene. The body's immune system reacted against these foreign particles, eventually destroying adjacent bone tissue and causing the implant to loosen - a condition called periprosthetic osteolysis.
Invention of the new material led to the discovery that high doses of radiation would "crosslink" the polyethylene, bonding molecules together to produce a much more durable material. The procedure also generates free radicals that could lead to oxidization and degradation of the implant, but the research team found that melting the material would eliminate free radicals. The first-generation highly cross-linked polyethylene was approved by the FDA for use in implants in 1999 and has been licensed to Zimmer, Inc.
However, the MGH researchers knew that the first-generation material had limitations in strength that made it unsuitable for some types of joint replacement implants. Subsequently, it was discovered that oxidation could be blocked by diffusing the antioxidant vitamin E throughout the polyethylene material. Both mechanical testing and animal studies have shown that the new material resists wear as well as the first generation and is much stronger. Vitamin-E-stabilized, highly cross-linked polyethylene has also received FDA approval for use in joint implants and has been licensed to both Zimmer and to Biomet, Inc., which made the implant used in the first surgical procedure in July 2007.
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